Idea Transcript
CHEM 438 FALL 2003
LAB 7
Laboratory 7 Gas chromatography of a Mixture at Two Different Temperatures Introduction to Gas Chromatography. GC is the method of choice for separating mixtures of volatile compounds. If they are nonvolatile or if they are thermally labile, then liquid chromatography (LC) is the preferred choice. GC uses a capillary column, which provides especially high efficiency. In any chromatographic separation, one has to select the appropriate separation conditions, including the choice of column type, gas flow rate, temperature, and whether to use a temperature gradient (i.e. a column temperature that changes during the separation). To a first approximation, compounds in GC separate according to their boiling point as they elute, so the choice of column type is not as critical as it is with LC. Gas flow rate is normally optimized by making a van Deemter plot, which was done is CHEM 120. Temperature is another important variable, and this is what we will explored in this lab. For this study, we choose a set of organic compounds, each of which is volatile and thermally stable. We will investigate the GC resolution of the compounds at two different temperatures. The chromatographic separations will be performed at a constant temperature, rather than using a temperature gradient., and why do we use capillary columns? Volatile compounds are approximately in equilibrium with the mobile and stationary phases, as they elute through the chromatographic column, and the ratio of concentrations in the two phases is described by the equilibrium constant, K, termed the partition coefficient, C (7-1) K= s Cm where Cs and Cm are the equilibrium concentrations of each analyte in the stationary and mobile phases, respectively. The retention time, tr, is directly related to the equilibrium constant K through a simple relation. V t −t K⋅ s = r 0 (7-2) Vm t0 where Vs and Vm are the volumes of the stationary and mobile phases, respectively, and t0 is the time that it would take an unretained compound to elute. Eq. 7-2 shows the linear relation between K and tr. One can imagine that K would increase with boiling point, since a higher boiling point will generally shift the equilibrium toward a greater concentration in the stationary phase (i.e. larger Cs) and a lower concentration in the mobile or gas phase (i.e. smaller Cm). Therefore, the retention time tr increases with boiling point. For the experiment, you will be given mixture that contains six volatile compounds whose properties are summarized in Table 7-1. Table 7-1. Properties of Compounds in the Unknown Sample Compound Molecular Weight Chemical Formula Methanol 32.04 CH3OH Toluene 92.15 C6H5OH Ethyl Benzene 106.17 C8H10 Para-xylene 106.17 C8H10 Meta-xylene 106.17 C8H10 Ortho-xylene 106.17 C8H10
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Boiling Point (ºC) 64.96 110.6 136.2 138.4 139.1 144.4
CHEM 438 FALL 2003
LAB 7
You will use a capillary column that is 30 m long and 0.25 mm inner diameter (ID), with a stationary phase consisting of a 0.25-µm film of an “innowax” crosslinked polyethylene glycol column. You will perform the separation of the mixture at two different temperatures, 60°C and 120°C, both at a gas inlet pressure of 15 psi. The gas for the mobile phase will be He. You will also have samples of the pure compounds that you can inject to help you identify the peaks by their tr values. You need only run the pure compounds at the lower temperature since we can reasonably assume that the order of their elution will not change. PRE-LAB ASSIGNMENT 1. Make a simple, neat, hand-drawn hypothetical chromatogram of two closely-spaced GC peaks and use it to explain what is meant by “resolution” in GC or any type of chromatography for that matter. Find an equation in your Chem 437 text (or any other Quantitative Analysis textbook) that describes how resolution is quantified. Label these parameters in your hand-drawn chromatogram. 2. Draw the chemical structure of all compounds in Table 7-1. You may use a hand-drawn sketch or software such as ChemDraw. 3. Predict which two compounds will be the most difficult to resolve and explain your logic. 4. Would you expect the peaks to come out earlier or later at the high temperature, and why? 5. Would you expect the resolution to improve or degrade at the high temperature? (This question is quite a bit harder, so explain your reasoning.) EXPERIMENTAL Record and print out your chromatograms of the individual components at the lower temperature. Record and print out your chromatograms of the mixture at the two temperatures. WRITTEN REPORT 1. Did the compounds elute in order of boiling point? Did temperature increase or decrease resolution? Was your prediction correct for the two compounds that you expected to be the most difficult to resolve? 2. Use your knowledge of basic thermodynamics to explain why the resolution improved or degraded at the higher temperature. 3. Use Excel to prepare a plot of Eq. 7-2. Plot K vs tr for different values of ratios of Vs/Vm and overlay the various curves. Use values of Vs/Vm equal to 10–8, 10–4 and 10–1. What do the trends indicate, and why do we use capillary columns? 4. If you wanted to rapidly monitor the levels of these compounds in an environment, and have reasonable accuracy, which temperature would you choose? There is no right or wrong answer, because it depends on how much speed and accuracy you need. But one answer is probably wiser than the other for this particular case. Explain the reasoning behind your choice. This lab was created by graduate student Ms. Stuti Christie and Professor Mary J. Wirth in Spring, 2003. It was revised by Professor Thomas P. Beebe, Jr., November, 2003. We welcome your comments on how to improve the learning experience of this lab. The best time to communicate these comments is when you are working on the lab.
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